Over many years harmful components of nitrogen oxides (NOx) have caused atmospheric pollution. NOx is contained in exhausted gases such as from internal combustion engines (e.g., automobiles and trucks), from combustion installations (e.g., power stations heated by natural gas, oil, or coal), and from nitric acid production plants.
Different methods have been used in the treatment of NOx-containing gas mixtures. One type of treatment involves catalytic reduction of nitrogen oxides. There can be two processes: (1) a nonselective reduction process wherein carbon monoxide, hydrogen, or a lower hydrocarbon is used as a reducing agent, and (2) a selective reduction process wherein ammonia or ammonia precursor is used as a reducing agent. In the selective reduction process, a high degree of removal with nitrogen oxide can be obtained with a small amount of reducing agent.
The selective reduction process is referred to as SCR process (Selective Catalytic Reduction). The SCR process uses catalytic reduction of nitrogen oxides with ammonia in the presence of atmospheric oxygen with the formation predominantly of nitrogen and steam:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+NH3→2N2+3H2O
A diesel engine exhaust contains phase materials (liquids and solids) which constitute particulates or particulate matter as well as NOx. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter, and a catalyst for the reduction of NOx.
The particulate matter emissions of diesel exhaust contain three main components. One component is a solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is a soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as a volatile organic fraction (“VOF”). The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. The VOF arises from two sources: (1) lubricating oil swept from cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel. A third component of the particulate matter is a sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel. Small proportions of SO3 are formed during combustion of the diesel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of total particulate matter.